To get a larger cargo hold, buy at least one ton of garbage and fly to a system where you won't get in trouble for jettisoning it. Then go to the "Cargo" screen and click slightly below the "garbage box". You'll notice the cargo capacity increases, but you still have the ton of garbage. Repeat this several times and you'll soon have enough space in the ship to accommodate larger weapons, for example. If you do this in a very small ship, you can buy a "Class 7 Hyperdrive" and travel very large distances across the galaxy with only 50 tons of fuel!